LFSR5 counter - libsidplayfp/SID_schematics GitHub Wiki
The LFSR5 counter further divides the clock during the decay/release phases to approximante an exponential curve.
It isbased on the circuitry we know from the LFST15 counter, it just looks a little bit more complicated:
Either LFSR5_clkS (shift) or LFSR5_clkH (don't shift, hold value) is active during sid_clk2.
Having that feedback path controlled by LFSR5_clkH was necessary because else the LFSR5 counter would lose the stored bits after some time without LFSR5_clkS being high.
Circuitry for generating the two clock signals LFSR5_clkS and LFSR5_clkH depending on if there was an "overflow" in the LFSR15 counter:
